Method for measuring the concentration of a substance in a solution

ABSTRACT

Methods and apparatus for measuring the concentration of decomposable substances, such as urea, are disclosed. The disclosed methods include adding a gaseous buffer, such as CO 2 , to the solution containing the decomposable compound, measuring the conductivity of the solution, decomposing the decomposable compound, measuring the conductivity of the thus-decomposed compound solution, and calculating the differential conductivities between the two measured solutions. The apparatus for carrying out these methods are also disclosed.

FIELD OF THE INVENTION

The present invention relates to a method and a device for measuring theconcentration of urea or a similar substance in a composite solution.The concentration of urea is difficult to measure directly, andtherefor, urea is catalytically decomposed by urease and thedifferential conductivity is measured.

The solution is preferably a medical solution but can also be abiological solution such as plasma. The invention relates particularlyto the measurement of the concentration of urea in connection withdialysis.

STATE OF THE ART

The present invention is based on the technique which is disclosed inEP-A2-0 437 789. This patent document discloses a system for measuringof for instance the urea concentration in a complex solution bydecomposing urea into ammonium ions catalyzed by urease. It is difficultto measure the urea directly and it must first be converted intoammonium ions. The change in the conductivity due to the contribution ofthe ammonium ions is measured by means of a conductivity meter.

In DE-C1-39 00 119 a urea sensor of similar type is described. Acapillary is positioned in a tube, through which blood passes. By meansof the capillary, plasma is drawn out of the blood and is allowed topass a urease column. The difference in conductivity before and afterthe urease column is measured and the difference is correlated to theurea content. In order to keep the temperature constant in the measuringapparatus the dialysis fluid is circulated in a closed loop around theurease column and the two conductivity measurement cells.

Document JP-60-165551 discloses a similar arrangement, where an ionexchange column is used to remove electrolytes by means ofanion-cation-exchange. In this way, the accuracy of the measured valuesare considerably improved since the relative change in the conductivitybecomes larger due to the fact that the initial conductivity is lower oralmost zero. Furthermore a buffer is added.

Document U.S. Pat. No. 3,930,957 describes a urea sensor, where anorganic buffer solution is added. As an example a solution of 0,05 MTris(hydroxymethyl)aminomethane can be used, which is adjusted to apH-value of about 6-7 by addition of glycine.

DISCLOSURE OF INVENTION

The object of the present invention is to provide a method and a devicefor measuring the concentration of urea, or a similar substance, in acomposite solution by heterogenous catalytic reaction of urea withurease in a reactor column for decomposing thereof, and measuring thedifferencial conductivity between reacted solution and unreactedsolution for providing an indication of the concentration of urea insaid solution. According to the invention, carbon dioxide is added tothe solution, which comprises hydrogen carbonate ions, before thereaction in the reactor column. The carbon dioxide, together with thehydrogen carbonate ions form a buffer maintaining the pH-value of thesolution within predetermined limits. At the same time, the carbondioxide contributes to making the relationship between the differentialconductivity and the concentration of urea linear over a large range.The addition of carbon dioxide also results in that each urea moleculeis decomposed into four ions, each contributing to the increase ofconductivity.

The carbon dioxide is added in gas form, and preferably in such anamount that the solution is substantially saturated with carbon dioxide.In order to further increase the solubility of carbon dioxide gas in thesolution, the pressure of the solution can be raised and/or thetemperature of the solution can be lowered. In this way it is assuredthat a sufficient amount of carbon dioxide is dissolved in the solutionfor making the relationship linear over as large a range as possible.

The differential conductivity between reacted and unreacted solution canbe measured by a single conductivity cell. The unreacted and reactedsolutions can be switched to the single conductivity cell in sequence.In this way the two measurements are as equal as possible independentlyof the construction of the conductivity cell.

It is also possible to measure the two conductivities with two separateconductivity cells, a first of which being positioned in a first branchincluding the reactor column and a second of which being positioned in abranch passing the reactor column. Since the measurement of conductivityis highly dependent on temperature, the measurement values must becorrected for temperature or the two different solutions must have thesame temperature.

By placing the two conductivity cells in close proximity to each otherand passing the two solutions through heat exchanging coils in a heatexchanger, the two solutions attain the same temperature.

Other features, properties and advantages appear from the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail below with referenceto the accompanying drawings.

FIG. 1 is a schematic view of a urea sensor of the type envisaged by thepresent invention.

FIG. 2 is a schematic view similar to FIG. 1 and shows the use of twoparallel conductivity meters.

FIG. 3 is a schematic view and shows valves switching one and the samemeasurement cell before and after the reactor column, respectively.

FIG. 4 is a valve diagram which shows the coupling sequence for thevalves according to FIG. 3.

FIG. 5 is a schematic view of a portion of FIG. 1 and shows the supplyof carbon dioxide gas into a sample conduit.

FIG. 6 is a schematic view of a portion of FIG. 1 and shows analternative method of eliminating bubbles of carbon dioxide gas.

FIG. 7 is a schematic view similar to FIG. 1 and shows the use of twoconductivity meters.

FIG. 8 is a schematic view similar to FIG. 1 and shows the use of a heatexchanger.

FIG. 9 is a detailed schematic view of a preferred embodiment of theurea sensor according to the present invention.

FIG. 10 is a cross-sectional view through the heat exchanger included inthe urea sensor according to FIG. 9.

FIG. 11 is an exploded view in perspective of a double conductivity cellpositioned at the bottom of the heat exchanger according to FIG. 10.

FIGS. 12 and 13 are schematic views similar to FIG. 9 and show the ureasensor during disinfection.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows a urea sensor or meter of the type to which the presentinvention relates.

A fluid or solution on which the measurement is to be carried out passesthrough a conduit 1. This solution can be a dialysis solution whichcomes directly or indirectly from a dialyser.

A sampling device 3 removes a partial quantity of the solution from theconduit 1 and transports this sample to an inlet conduit 2. The samplingdevice is exemplified in FIG. 1 by a pump 6.

A charging device 4 is connected to the inlet conduit 2 for the additionof one or more substances. The addition is controlled by a pump 5 inFIG. 1.

The sample solution is supplied to a reactor column 10 and is allowed topass through this. The reactor column can contain urease for catalyzingthe decomposition of urea into ammonium ions and bicarbonate ions.

The sample is fed out from the reactor column 10 to a measuring device 7which measures the decomposed product in the reactor column. Themeasurement device 7 can be a differential conductivity cell whichmeasures the change in conductivity before and after the reactor column10 respectively.

The sample is fed out from the measuring device 7 to an outletarrangement 8.

An ion exchanger can be included in the inlet conduit in order to lowerthe content of undesirable ions, thereby to improve the accuracy of themeasuring device.

The invention mainly relates to the measurement of urea and the reactionin the reactor column is linearised by using addition of carbon dioxide.

The solubility of carbon dioxide is improved by a high pressure and atleast the conductivity measurement can take place at increased pressure.

By lowering the temperature, the solubility of carbon dioxide increases,which also can be used with advantage in the present invention.

Solution

The composite solution which is present in conduit 1, and where thecontent of a substance is to be measured, is a solution of a medicaland/or biological type.

In a preferred embodiment, the solution is a dialysis solution, the ureaconcentration of which is to be determined. A dialysis solutioncomprises electrolytes in for example Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻⁻, HCO₃⁻⁻, CH₃ COO⁻ in predetermined concentrations and combinations as well aspossibly further substances such as glycose. When the dialysis solutionpasses a dialyser an exchange of low-molecular-weight substances occursbetween blood on one side of the membrane of the dialyser and thedialysis solution on the other side of the membrane. Various substancesthereby pass from the blood into the dialysis solution, such as urea,creatinine, etc. At the same time certain substances pass from thedialysis solution into the blood, such as bicarbonate ions HCO₃.sup.·.

Other types of solutions where the present invention can be applied areblood, in which high-molecular-weight substances are preferably firstseparated, for example by means of a membrane, after which theultrafiltrate, i.e. blood plasma, is analyzed.

Other types of solutions which can be used according to the presentinvention are dialysis solutions which are used for peritoneal dialysis,whereby the out-going PD-solution is analyzed.

Additional other solutions which can be analyzed are urine, sweat, tearfluid, saliva or other extra-cellular fluids which are sucked outthrough the skin or withdrawn in another way.

The solution in the conduit 1 can also be a fresh dialysis solution orinfusion solution where the concentration of a particular substance isto be determined, for example glycose or penicillin.

The substances of which the concentration is to be measured in thesolution are preferably such substances which require catalyticdecomposition in order to be measured, and particularly those whichrequire a pH-value lying within a small range, i.e. requiring thepresence of a buffer system. Examples of such substances are: urea,L-glutamine, L-citrulline, N-acylaminoacid, penicillin, L-asparagine,cholesterol, glycose. These substances can be made to decompose or toreact during enzymatic influence in the reactor column 10 via knowncorresponding enzymes. For a more detailed determination of suitablecombinations reference is made to EP-A2-0 437 789 and U.S. Pat. No.4,311,789.

The invention is described below with reference to urea and urease, butis also applicable to the aforementioned substances and other similarsubstances.

Sampling Device

A sample is taken out from the solution in conduit 1 by means of thesampling device 3. In a preferred embodiment the sampling device is apump 6 which is driven so that a constant small flow is taken out fromthe conduit 1. The sample flow can be between about 0,1-10 ml/min,preferably about 0,5-5 ml/min, such as for example 1 ml/min.

The sampling device can also be driven intermittently so that a sampleis taken out per unit of time, for example at an interval in the regionof 5-60 minutes, for example 30 minutes. The volume of the sample canthereby be between about 1-100 ml/sample, preferably about 5-50ml/sample, such as for example 10 ml/sample.

In one embodiment, the pump is driven in the sampling device 3 so thatthe sampling flow to the inlet conduit 2 is proportional to the flow inthe solution conduit 1 with a particular proportionality constant. If,for example, the solution in the conduit 1 is a dialysis solution whichpasses at about 500 ml/min, a five-hundredth part is taken out via thesample device 3, i.e. about 1 ml/min. If the flow in the conduit 1varies, the flow in the inlet conduit 2 also varies. The advantage withthis embodiment is that the present invention can thereby be combinedwith the invention which is disclosed in European patent application EP94.102383.0. The pump of the sampling device 3 can also be drivenintermittently but each time so that the extracted sample flow is aparticular proportion of the solution flow in the conduit 1.

The pump 6 can be a ceramic pump with constant displacement perrevolution and can be of the same type which is used in the monitorGAMBRO AK 100. Thus a very accurate metered amount can be taken out fromthe conduit 1. Alternatively a peristaltic pump of known type can beused, or other similar pumps.

In the case that the content in the conduit 1 consists of blood the sametechnique can be used as described in DE-C1-39 00 119 where a hollowfibre of the same type as used in a hollow fibre dialyser is used forthe sampling. The required under-pressure for taking out theultrafiltrate, i.e. plasma, is created by the pump.

It is also possible to perform the sampling from a container comprisingthe solution to be analyzed. In this way the container replaces theconduit 1.

Charging Device

The sample which is taken out by means of the sampling device 3 is fedto the inlet conduit 2. An additive may possibly be added in thisconduit. In certain cases it is desirable to adjust the pH-value for thesolution so that the desired reaction is obtained in the reactor column10.

In the preferred embodiment the solution which is to be analyzed is adialysis solution which contains urea and has a pH-value normally ofaround 7,4. The decomposition of the urea to ammonium ions brings aboutan increase in that pH-value.

U.S. Pat. No. 3,930,957 describes the addition of an organic buffer inliquid form. An alternative liquid buffer system which can be used is aso-called phosphate buffer consisting of H₂ PO₄.sup.· /HPO₄ ²· which hasa pH approximately equal to 7.

One disadvantage with a buffer in liquid form is, however, that itdilutes the solution and adds ions and thus alters the conductivity ofthe solution before the reaction in the reactor column.

According to a preferred embodiment of the present invention a buffer ingas form is used, namely carbon dioxide (CO₂), which cooperates with thebicarbonate buffer already present in the dialysis solution. In thisconnection carbon dioxide is supplied in gas form from the chargingdevice 4 via the pump 5 to the inlet conduit 2. This is described inmore detail below.

The use of carbon dioxide gas as an additive for the dialysis solutioncomprising bicarbonate and for application to measuring the conductivitygives at least two advantages relative to a buffer in liquid form,namely that the additive of carbon dioxide does not give any volumechange and thus no dilution of the solution and also that the dissolvedcarbon dioxide has no inherent conductivity.

By use of a buffer system in connection with a dialysis solutioncontaining urea, in particular carbon dioxide gas, the pH-value can becontrolled so that optimal effect is obtained from the urease. At thesame time precipitation of calcium carbonate is avoided.

Reactor Column

An embodiment of the reactor column 10 is shown in FIG. 7 and comprisesa cylindrical container which contains the enzyme which is required. Inthe preferred embodiment the column 10 comprises urease 11 which isimmobilized by means of aluminium oxide 12 granules. Closely-meshedfilters 13, 14 are arranged at the inlet and the outlet. The filtersprevent the urease and aluminium oxide, if released, from passing out ofthe column 10. The filters furthermore prevent larger particles fromentering the column.

The cylindrical container 10 has to have a sufficiently large volumesuch that the substance which is to be transformed is able to come intocontact with, or close to, the corresponding enzyme for the length oftime that a decomposition reaction is catalyzed. The column ispreferably arranged so that the sample solution comprising the substancereaches the activation proximity of the enzyme to at least 99% for aslong as the corresponding reaction is catalyzed.

It is preferred to feed the solution in from below via an inlet 15 andto feed out the reacted solution at the upper end of the column via anoutlet 16. Additionally, a filter 17 is provided which separates anupper part of the column 10, which only contains aluminium oxide 18,i.e. without urease.

It is clear that the column 10 can have various constructions dependingon which substance is to be analyzed and which enzyme is used. Thus thecolumn may also be horizontal or the flow can be reversed and pass fromthe top downwardly. Additionally, it is possible with intermittentfunctioning to introduce the sample in the column 10 and to let thisremain in the column for as long a time as a reaction is obtained, afterwhich the sample is fed out for analysis and measurement.

In said preferred embodiment protein and fat are included with thedialysis solution. It is desirable to separate protein and fat before orafter the column, whereby one or both of the filters 13 or 14 mayconstitute a double filter, where one half of the filter has the task offiltering the incoming or outgoing solution and retaining the ureasewhilst the other half of the filter constitutes a cellulose filterimpregnated with active carbon. In such a filter organic molecules areabsorbed, practically speaking, completely. Other filter designs canalso be used.

Measuring Device

The measuring device 7 is dependent on the substance which is to beanalyzed in the solution. In connection with the preferred embodimentwhere urea is to be analyzed, the measuring device 7 can be an ammoniumion-sensitive electrode which determines the ammonium ion content in thesolution after the reactor column 10. Such a measuring device isdisclosed in WO 94/08641 and U.S. Pat. No. 4,686,479.

The measuring device can also be a pH-meter or a meter for gas which isreleased, such as ammonia, see WO 93/22668.

According to a preferred embodiment the measuring device is however anarrangement for measuring the difference in conductivity of the solutionbefore and after the urease column 10.

With reference to FIG. 1, the reactor column 10 is preceded by athree-way valve 20, by means of which the inlet flow can be diverted toa shunt conduit 9 and a second three-way valve 23 past the reactorcolumn 10. When the three-way valves 20 and 23 are in the positionsopposite as shown in FIG. 1 the dialysis solution passes beyond thecolumn 10 via shunt conduit 9 directly to the measurement cell 7 formeasurement of the initial or unreacted conductivity of the dialysissolution, i.e. without decomposing the urea. Then the three-way valves20 and 23 are switched to their other position shown in FIG. 1 so thatthe solution can pass through the reactor column 10 and further to themeasurement cell 7 and a reaction conductivity value is measured. Bythis embodiment, the same measurement cell 7 is used for measuring bothunreacted and reacted conductivity values, which is beneficial.

In order to carry out the measurement even more reliably it may besuitable to introduce a delay conduit 22 in the shunt conduit 21 withthe same volume as the reactor column 10, which is shown in FIG. 2. Thedelay conduit 22 preferably comprises the same amount of aluminium oxideas the urease-column 10 so that account is taken of the contribution ofaluminium oxide to the conductivity. In the embodiment of FIG. 2, twodifferent conductivity cells are used, which however can be calibrated,see further below.

A further method for determining the conductivity before and after thereactor column respectively is to arrange a measurement cell before anda measurement cell after the column respectively, which is shown in FIG.7.

A preferred measurement method is disclosed in FIG. 8 and involves thesample solution being divided into two parallel flows and measurementsbeing carried out on respective flows with separate measurement cells inorder to simultaneously determine the initial conductivity and thereaction conductivity. It will suitably take approximately the same timefor the sample solution to reach the respective measurement cell so thatthe two measurements are carried out on approximately the same sample.

A further way is to use the same measurement cell for measuring theconductivity on the same specific sample not only before but also afterthe reactor column 10. FIG. 3 shows the reactor column 10 and ameasurement cell 7 coupled together with a plurality of valves 31a, 31b,32a, 32b, 33, 34, 35 and 36. The valves are operated according to thevalve scheme in FIG. 4, where O defines open valve and C defines closedvalve. The switching scheme has twelve steps denoted T1-T12. After thisthe same cycle is adopted again.

At time T1 the valves 31a, 32a, 33 and 34 are open and the flow firstoccurs through the measurement device 7 and thereafter through thereactor column 10. This position is maintained during a relatively longtime period so that the reactor column 10 and the conduits are filledwith a sample and an initial conductivity is measured.

At time T7 the valves have been switched so that the valves 31b, 32b, 35and 36 are open. In this position the sample which earlier passed themeasurement device 7 into the reactor column 10 flows through the samemeasurement device 7. Also this position, T7, is. maintained during asufficiently long time so that a reliable measured value of the reactionconductivity can be obtained from the measurement device 7.

In this way the same sample or amount of fluid passes through themeasurement device 7 twice (although in different directions) andinbetween has passed the reactor column 10. A particularly accuratemeasurement can be obtained in this way.

The different switching steps T2-T6 as well as T8-T12 have the task ofremoving pressure pulses in connection with the switching due to thefact that the measurement device 7 is coupled in parallel with shuntconduits 37, 38 during the switching process. It is of course possiblethat other valve schemes can be used than that shown in FIG. 4.Similarly the discrete valves 31-36 may be replaced by three-way valveswhich perform similar functions. It will be clear to a skilled man howsuch valves should be connected with the guidance provided by FIG. 3 andFIG. 4.

In a preferred embodiment the measurement device 7 is a conductivitymeasurement cell which measures the conductivity of the solution whichpasses the measurement device.

The conductivity measurement cell can be of so-called four-pole typewhere the electric voltage is supplied to two feed electrodes positionedat a distance from one another. Two detector electrodes are positionedtherebetween. The voltage over the detector electrodes is measured andthe voltage which is supplied to the feed electrodes is regulated sothat the measured voltage is constant. The resultant current is measuredand is proportional to the conductivity of the solution. By means offour-pole measurement it is avoided that transition resistances betweenthe supply electrodes and the solution affect the result. The feedvoltage is an alternating voltage.

A conductivity measurement cell has a relatively large temperaturedependence, for which reason it is necessary to maintain the temperatureconstant and/or correct for temperature variations as described in moredetail below.

For calibration purposes or for taring, a shunt conduit 24 is shown inFIG. 2, said shunt conduit connecting the inlet of the two conductivitycells 7a and 7b with each other. By coupling-in this conduit theconductivity cells can be balanced to give the same measured value. Ifthe conduit 24 is double the conductivity cells can be cross-connectedfor comparison of the measured values.

Outlet Arrangement

The sample is fed from the measurement device 7 to an outlet arrangement8, which in FIG. 1 is shown in the form of a collection vessel.

Alternatively, the sample solution can be sent to a drain or be fed backto the conduit 1 downstream of the sampling device 3.

If the present invention is to be combined with the invention accordingto EP 94.102383.0 as mentioned above, an extra three-way valve 25 isused, said valve being positioned in the shunt conduit 9 (see FIG. 1),or after the conductivity cell 7b in the corresponding conduit in FIG. 2to the left where the solution has not passed the urease-column. Inorder to ensure that a certain proportion of the dialysis solution inthe conduit 1 passes through the shunt conduit, a separate pump 26 isused (see FIG. 2). The solution is led from the three-way valve 25 to aseparate collection bag 27. The volume of solution in the collection bag27 thus has a predetermined relationship to the amount of solution whichhas passed through conduit 1, for example 1:500. Furthermore theconcentration of the substances included in the collection bag is thesame as the average in the conduit 1. By integrating the measured valuesfor the urea sensor over the dialysis time, a total value of the urea isobtained which can be compared with the concentration in the collectingbag (which is analyzed in a different way). In this manner the functionof the urea sensor can be monitored and double safety is achieved. Sincethe urea has been decomposed in the right branch, the solution which haspassed along this route cannot be used for this purpose.

Ion Exchanger

In certain cases it is desirable to remove electrolytes from the samplesolution before it is fed into the reactor column 10 and the measuringdevice 7. It is the difference between the conductivity before and afterthe reactor column 10, which is measured. If the measured value beforethe reactor column 10 is low or zero, a better accuracy is obtained. Forthis reason an ion exchanger can be incorporated into the inlet conduit2 before the reactor column 10, as shown in FIG. 1 with dashed lines at21.

Such an ion exchanger can be of conventional construction, where theions are exchanged with corresponding hydrogen ions or hydroxide ionswhich form water. Such ion exchangers can be very effective but usuallyrequire a large amount of space.

Other methods can also be used in order to minimize or eliminate theelectrolytes in the solution before the reactor column 10. One exampleis to use an electrostatic method, where the solution in the inletconduit 2 is made to pass charged electrode surfaces and where thecharged ions are attracted by the electrodes' surfaces whilst theuncharged substances, such as urea, are unaffected.

Additional other known ion exchange techniques can be used.

Urea

The present invention is particularly intended for measuring the ureaconcentration in a dialysis solution after a dialyser. The ureaconcentration in this dialysis solution is related to the urea contentin the blood which is cleaned by the dialyser.

Urea is often used as an indicator for whether an adequate dialysis hasbeen obtained. Exactly how the urea measurement on the dialysis side isrelated to the urea concentration in blood and how this is interpretedrespectively in order to determine whether the required dialysis hasbeen obtained, is not the subject matter of this invention and will notbe described further. There are a large number of literature articleswhich discuss these questions and attention is directed to EP-A1-0 547025 as well as WO 94/08641.

As concerns measurement of the urea concentration, this cannot bemeasured directly in an easy manner. As described above the solutionwhich comprises urea must pass a reactor column 10 which comprisesurease. Urease is an enzyme which catalyzes the transformation of ureato ammonium ions according to the reaction (1) below: ##STR1## Thusammonium ions and ammonia are formed as well as hydrogen carbonate ions(bicarbonate). The yield is practically 100% if the contact between theurea and the urease is sufficiently effective.

On the other hand ammonia is decomposed into ammonium ions in dependenceon the pH-value of the solution, according to the equilibrium reaction(2) below:

    NH.sub.3 +H.sub.2 O˜NH.sub.4.sup.+ +OH.sup.·(2)

If the solution comprises a buffer system such as the phosphate buffermentioned above, this equilibrium (2) will be replaced at sufficientlylow pH-value, for example at pH=7, by the following reaction (3):

    NH.sub.3 +H.sub.2 PO.sub.4.sup.· →NH.sub.4.sup.+ +HPO.sub.4.sup.2·                                (3)

With the aforementioned phosphate buffer the pH-value can be adjusted toapproximately 7 and the amount of buffer regulated so that this pH-valueis shifted only a very tiny amount. In this way, by suitable choice ofthe pH-value, practically all the ammonia can be transformed intoammonium ions. Other buffer systems can also be used such as disclosedin U.S. Pat. No. 3,930,957.

The disadvantage with using a buffer of the phosphate buffer type isthat it dilutes the solution and contributes to the conductivity beforetransformation in the reactor column, which makes the measurement of thedifferential conductivity additionally difficult. This problem istreated in U.S. Pat. No. 3,930,957 which proposes buffered organiccarrier solutions which in themselves have very low electricconductivity and which do not react with the enzyme used.

Carbon Dioxide

In a preferred embodiment according to the present invention it isproposed that a buffer system consisting of carbon dioxide and hydrogencarbonate (bicarbonate) ions be used. Particularly with the measurementof the urea concentration in a dialysis solution, great advantages canbe obtained. The dialysis solution normally comprises bicarbonate whichtogether with carbon dioxide forms said buffer system.

Carbon dioxide reacts with ammonia according to the following reaction(4):

    NH.sub.3 +CO.sub.2 +H.sub.2 O→NH.sub.4.sup.+ +HCO.sub.3.sup.·                                 (4)

The reaction upon which the present invention is based is the sum ofreaction (1) and reaction (4), which results in the following reaction(5):

    (NH.sub.2).sub.2 CO+CO.sub.2 +3H.sub.2 O→2NH.sub.4.sup.+ +2HCO.sub.3.sup.·                                (5)

As is clear from the reaction (5) two ammonium ions and two hydrogencarbonate ions are obtained from each urea molecule, all four of whichions contribute to the increase in conductivity. Since carbon dioxide isadded in excess, reaction (5) is displaced to the right so that theexchange becomes practically 100%. By addition of surplus carbon dioxideit is further obtained that the pH-value for the solution will berelatively low, which will prevent the precipitation of calciumcarbonate.

If the pH-value is under about 7,35, the exchange in the above reactionis larger than about 99,5%. Additionally, the activity of urease islargest within the pH-range of about 6 to about 8. If at the same timethe residence time for the solution in the urease column is sufficientlylong, a total exchange in excess of 99% is obtained. Thus a pH-value forthe solution after the urease-column lying between about 6 and about 8is preferred, preferably between 6,2 and 7,4.

The carbon dioxide is added by means of the charging device 4. A methodfor adding carbon dioxide gas to the sample solution in the conduit 2 isshown in more detail in FIG. 5.

The charging device comprises a source of carbon dioxide under pressure41 as well as a pressure reduction valve 42. Carbon dioxide gas is fedfrom this source with a predetermined pressure to a conduit 43. Theconduit 43 is provided at its lower end with a connector 44 which passesthrough the wall in the conduit 2 into the sample solution.

A silicon tube 45 or other gas-permeable tube is connected to thecoupling 44, said tube being positioned in the conduit 2 substantiallyconcentrically therewith along a predetermined length. The silicon hasthe property that carbon dioxide gas present within the tube 45 can bediffused through the silicon material and emitted to the solution in theconduit 2.

The regulation of the pressure regulator 42 is controlled by twopressure meters 46, 47, which detect the pressure difference over thetube 45. These two pressure meters can of course be combined into onedifferential pressure meter.

The diffusion speed is dependent on the differential pressure across thetube 45. By adapting the length of the tube as well as the pressuredifference, the desired amount of carbon dioxide can be fed into thesample solution in the conduit 2. Preferably as much carbon dioxide isadded so as to make the sample solution substantially saturated. In thiscontext there is normally a sufficient amount of carbon dioxide whichcan be used in reaction (5) in order to decompose all the urea toammonium ions.

At high urea concentrations it is difficult to add a sufficient amountof carbon dioxide gas which will dissolve in the sample solution. Inthis case a surplus of carbon dioxide gas can be added, said gas beingincluded with the sample solution in the form of micro-bubbles or moreor less large bubbles. This gas dissolves partially in the samplesolution during transport to the urease column and the remaining amountof carbon dioxide gas is used during the reaction in the urease column.

FIG. 7 shows a further method of supplying carbon dioxide gas to thesample solution in the conduit 2. As in FIG. 5, a source 41 of carbondioxide gas under pressure is provided. Additionally two valves 49 and50 are provided, between which a container 48 with predetermined volumeis placed. The valve 49 connects the container 48 with the source 41 andthe valve 50 connects the container 48 with the conduit 2. Two pressuremeters 51a and 51b are arranged across the valve 50.

The function of the charging device according to FIG. 7 is according tothe following. The valve 49 is opened and carbon dioxide is fed from thesource 41 to the container 48 until the pressure meter 51a detects apredetermined pressure. The valve 49 is then closed and the valve 50opened, whereby the contents in container 48 is supplied to conduit 2.

When the pressure meter 51a or 51b detects a low pressure, the valve 50is closed and the valve 49 is opened for a renewed filling of container48.

Since the volume of the container 48 and the pressure difference areknown, the supplied amount of carbon dioxide in the conduit 2 can bedetermined.

The valve 50, in the open condition, can perform a certain throttling sothat supply of carbon dioxide gas occurs with a relatively even speed.By regulating the degree of throttling in the valve 50 during its opentime, the amount of gas supplied per unit of time can be regulated.

It is also possible to operate the valve 50 so that a large bubble ofcarbon dioxide gas is fed into the sample conduit 2 so that the bubblefills up the complete cross-section of the conduit 2. During thefollowing transport towards the reactor column, the bubble of carbondioxide gas dissolves partially in the sample solution next to it sothat this becomes substantially saturated with carbon dioxide. Theremaining amount of carbon dioxide gas is finally dispersed by thefilter 13 in the reactor column 10 (see FIG. 2) so that the carbondioxide gas is evenly distributed in the sample solution. The inletconduit 15 should thereby be conically shaped so that the samplesolution is divided over the whole cross-section of the reactor column10. During passage through the reactor column 10, the carbon dioxide isused in reaction (5), whereby the surplus of carbon dioxide is used up.

In order to ensure that no carbon dioxide in gas form remains in thesolution which leaves the reactor column 10, which would disturb thefollowing conductivity measurements, the outlet conduit 16 (see FIG. 7)may be provided with a bubble detector 52, for example an ultrasounddetector. The output signal from one such bubble detector can be used inmany ways. It can be used to indicate feasibly erroneous measurementvalues and/or to regulate the supply of carbon dioxide gas. In thelatter case it should be observed that the time delay from supply todetection is relatively long, for example four minutes. If theconcentration of urea does not change too quickly, an efficient andoptimal regulation can however be obtained.

In connection with the bubble detector 52 a gas separator 62 (see FIG.6) may be present, separating any possible gas in free form from thesolution in outlet 16.

The carbon dioxide gas source 41 is preferably formed by a pressurecontainer with carbon dioxide gas in condensed form, which immediatelyvaporizes upon release and corresponding reduction of pressure. Suchcarbon dioxide cartridges are available in different sizes. Theconsumption of carbon dioxide gas is ever so small that such a carbondioxide cartridge of small dimension will suffice for very long periodsof operation.

An alternative source of carbon dioxide gas can be the production ofcarbon dioxide gas in-situ. One way is to heat sodium bicarbonate powder(NaHCO₃) to a high temperature, for example over 50° C. In this way thesodium bicarbonate is decomposed and carbon dioxide gas is given off.The bicarbonate can be provided in a small container which is placed ona heating element. When the apparatus is to be used, the heating sourceis activated and the carbon dioxide gas is given off after a short time.The cartridge is of course replaceable. Other methods are known in theart. The exact method of manufacturing carbon dioxide gas does not formpart of the subject matter of the present invention, but reference ismade to EP-A1-0 481 257 where a method for production of carbon dioxidegas is described.

During the reaction in the urease column 10 the pH-value of the solutionrises, particularly if the urea concentration is high and the content ofcarbon dioxide gas is too low. If the pH-value rises above about 7,4there is a risk for precipitation of calcium carbonate and magnesiumcarbonate (calcium ions and magnesium ions are normally part of thedialysis solution). If such precipitation occurs in the measurement cell7, this can very soon lead to erroneous measured values and themeasurement device must be cleaned.

In order to obtain a sufficiently low pH-value in the solution whichpasses the measurement cell 7, it can be desirable to add carbon dioxidegas also at the outlet from the urease-column 10, i.e. at the outlet 16.

It is also possible to provide the column 10 with a second inlet forcarbon dioxide approximately at the filter 17 (see FIG. 7) or in otherplaces, whereby it is ensured that the pH-value is always sufficientlylow to avoid precipitation of calcium carbonate in the conductivitymeasurement cell.

Preferred pH-values (at a position after or inside the conductivitycell) are between 5,5 and 8,5, preferably between 6 and 8. With use ofcarbon dioxide and bicarbonate as the buffer, it is preferred that thepH-value lies between about 6,2 and about 7,7, preferably between 6,3and 7,4.

Pressure

It is known that the solubility of carbon dioxide gas in water isessentially proportional to the pressure in the water. This fact is usedin the embodiment shown in FIG. 7.

An adjustable throttle valve 53 is arranged after the outlet 16 of theurease column. The throttle valve 53 functions together with the pump 6in order to raise the pressure in the conduit 2 and the urease column aswell as the measurement device 7a if bubbles are detected by the bubbledetector 52. The carbon dioxide gas then dissolves due to the increasesolubility. The pressure is suitably re-adjusted to normal pressure assoon as the supply of carbon dioxide has been reduced so that no bubblesremain.

In another embodiment the throttle valve 53 is used together with themeasurement device 7a in order to detect the occurrence of bubbles inthe sample solution which upsets the measurement in the measurementdevice 7a, like a replacement for, or complement to, the bubble detector52. At predetermined intervals, where it is desirable to check whetherthe measured results are reliable, the pressure is raised momentarily byactivating the throttle valve 53. The pressure increase can be monitoredby the pressure meter 51b or a separate pressure meter. If the pressurerise results in a change in the measured value in the measurement device7a, this is a sign that the sample solution contains gas bubbles whichdisturb the measuring. This information can be used in order to reducethe supply of carbon dioxide gas, after which a new test is carried outafter a certain time in order to verify that the change has given thedesired effect.

The pressure meter 51b can also be used in order to ensure that thesample solution 2 is always at approximately the same pressure, forexample atmospheric pressure. If the sampling occurs at the outlet froma dialysis machine, the pressure before the pump 6 can varyconsiderably. If the discharging device 8 is formed by a return conduitto conduit 1, the whole of the sample conduit 2 will be at that pressurewhich is present in the conduit 1. By manoeuvring the throttle valve 53and the pump 6, which is monitored by means of the pressure meter 51b,the desired pressure in the sample conduit 2 as well as the measuringdevice 7a can be set, for example at somewhat above atmosphericpressure.

The aforementioned pressure increase only effects the measured result inthe measurement device 7a to a very small extent in normal cases if themeasurement device 7a is a conductivity cell. With the use of othertypes of measurement devices, one has to correct for thepressure-dependence of these measurement devices when applying theaforementioned method with pressure increase.

As an alternative to the method described above a local pressureincrease in only the measurement device 7a can be used. For this anextra pump arranged before the measurement device 7a and a throttlevalve arranged after the measurement device 7a are used. The extra pumpand throttle device 53 are controlled so that the pressure in thereactor column 10 is not affected. Apart from this the function is thesame as described above.

It is also possible to use a permanent pressure increased. The pressureincrease could be in the range 0,03-0,3 MPa, preferably about 0,1 MPa.In this way a sufficient amount of carbon dioxide gas can dissolve inthe solution in order to satisfy the reaction (5) and at the same timeto ensure an outlet pH-value below about 7,4 up to a urea concentrationof about 35 mM, without extra addition of carbon dioxide gas.

Temperature

It is known that the logarithm of the solubility of carbon dioxide gasin the sample solution in the conduit 2 is inversely proportional to thetemperature. Normally the temperature of the used dialysis solution isabout 36-37° C.

In order to avoid dependence on the temperature in the incoming dialysisfluid, the sample solution can be heated up, by means of a heatingelement, to a fixed temperature which lies at a value between about 38and about 45° C., preferably about 40°. The reason that the temperatureis chosen to be so high is that it is higher than all expected incomingtemperatures, for which reason no cooling is required and consistentmeasurements are obtained independent of the incoming temperature.

It is desirable to maintain the temperature substantially constantduring the passage of the sample solution through the device, i.e. fromthe sampling device 3 through the reactor column 10 and the measurementdevice 7. At least the temperature of the reactor column should beconstant in order to obtain uniform activity. However the temperatureshould not be raised (at least not too much) before the conductivitymeasurement cell in order to avoid possible bubble formation due tosurplus carbon dioxide.

According to one embodiment of the present invention the temperature isinstead lowered on the incoming sample solution by means of cooling.Such cooling can occur in many different ways, but since the amount ofsolution which is to be cooled is relatively small, Peltier-elements canbe used where the electric current is directly converted into cooling.The sample solution is made to pass a Peltier-element 55 and current issupplied so that the required temperature drop is obtained as shown withdashed lines in FIG. 6, which is detected by a temperature sensor 56positioned downstream of the Peltier-element 55. A suitable temperaturein this alternative embodiment is about 25° C.

The temperatures used should lie in the range of about 20° C. to about50° C. The reason for this is that below about 20° C. the urease-enzymehas low effectiveness, which means that the transformation from urea toammonium ions takes too long time. Above about 50° C. the urease-enzymeis decomposed, which of course is undesirable. A temperature betweenabout 25° C. and about 45° C. is therefore preferred.

By using lowering of temperature with Peltier-elements, it is ensuredthat a sufficient amount of carbon dioxide gas can always dissolve inthe sample solution in the conduit 2.

A lowering of the temperature in connection with the measurement cellcan be used in order to detect the presence of carbon dioxide gas whichupsets the measuring, analogous with the aforementioned pressureincrease. With lowering of the temperature in the measurement cell thesolubility of carbon dioxide increases and carbon dioxide gas in bubbleform will possibly dissolve. In order to compare conductivity valuesbefore and after such a temperature reduction, temperature compensationof the measured values is however required which can be difficult toachieve with desired accuracy.

FIG. 6 shows a gas trap 62 in the form of a gas-permeable tube 57 ofsilicon. Outside the tube there is a solution with a low content ofcarbon dioxide, for example dialysis solution, such as the content inthe heat exchanger as described in connection with the preferredembodiment in FIG. 8 (see below). The surplus of carbon dioxide gasinside the tube 57 may possibly be given off to the fluid outside thetube 57, particularly if a pressure difference is present. Other typesof gas separators can also be used.

Priming

The urease-column 10 contains a dry powder which has to be moistenedbefore use. This can take place by allowing a physiological sodiumchloride solution to pass through the urease-column and the measurementdevice 7.

Alternatively one can use the dialysis solution which is used by thedialysis machine during its start-up period in order to moisten theurease column and put this into an operation-suitable condition. Thedialysis machine carries out a start-up procedure called "priming",whereby the dialysis solution is circulated through the dialysismachine's system without the dialyser which is shunted. This solutioncan also be used for priming the urea-sensor.

Disinfection

Normally no particular cleaning of the urea-sensor is required. It ishowever recommended to flush out the urea-sensor after completed use,for example by allowing a physiological sodium chloride solution to passthrough the urea-sensor for a predetermined time. A disinfection withraised temperature is normally not required. However the disinfection ofthe dialysis monitor and the urea sensor can be coordinated so that thedisinfection solution may also pass through the urea sensor. In thiscase a heat disinfection is preferred, but also disinfection usingcitric acid etc. may be used (CleanCart).

In certain cases the dialysis solution contains glycose which can causeproblems. By flushing after use such problems are however avoided.

The urea sensor according to the invention can be used completelyseparate from a dialysis monitor. In this case the urea sensor isconnected to the outlet tube of the monitor. Of course the urea-sensorcan be fixed to the casing of the apparatus and connected at a differentlocation, for example immediately after the dialyser. It is alsopossible to completely integrate the urea sensor into the dialysismachine and/or provide the urea sensor with an electronic interface tothe dialysis machine.

Quality Test

Before using the urea sensor it is preferable to establish thateverything is working satisfactorily. To this end, a small amount ofsolution containing urea at a particular known concentration is suppliedto the conduit 2 and may pass the urea sensor. If the measurement device7 does not produce an expected measured value it has to be exchanged andsent for repair/service.

Such a quality control can be carried out in a relatively simple way inthat the urease column 10 comprises a small amount of urea in powderform at its inlet 15. As soon as the physiological sodium chloridesolution or other solution is supplied for priming, the urea powder isdissolved and transformed by the urease column 10 into ammonium ionswhich give a reading on the measurement device 7. In this way only oneexchangeable and disposable unit is required, namely the urease column10 containing said urea powder.

The urease column 10 consists of a cartridge which is exchangeable.Before a dialysis treatment a new cartridge is put into place andpriming occurs as described above.

With the case of manufacturing carbon dioxide gas in-situ, a secondcartridge containing the necessary ingredients can be used. These twocartridges can form a unit by being built together in a suitable manner.The unit is exchanged in connection with each measurement and isdimensioned to allow measuring during a normal dialysis treatment, whichcan be about four hours with haemodialysis.

PREFERRED EMBODIMENT

FIG. 8 shows a preferred embodiment of the invention. A sampling pump 6extracts a sample solution from the content in the conduit 1. Thissample solution is divided into two parallel branches with approximatelyequal amounts in each branch. The first branch (to the right in FIG. 8)contains the urease-column 10 as well as the supply of carbon dioxidefrom a carbon dioxide source 41 via a dosage valve 60. The second branchcontains a delay conduit or a delay vessel 22 which contains the samequantity of aluminium oxide as the urease-column 10 and additionally hasthe same flow resistance as the urease-column 10, but has no urease.

The contents of the urease-column 10 and the delay conduit 22 aresupplied to heat exchanger coils 71, 72 of a heat exchanger 70, whichcontains a large amount of fluid. The content of each branch, 71 and 72,is transferred to each conductivity cell 7a and 7b, respectively. Afterthe conductivity cells the two branches are combined to a single outletconduit 73, in which a variable throttle device 53 is arranged andfinally the sample solution is given off to a collection container 8 (orto conduit 1).

The inside of the heat exchanger is filled with a fluid which is kept ata very constant temperature. This fluid can be the dialysis fluid whichpasses in the conduit 1 or it can be another fluid which is heated to adesired constant temperature. In order to regulate the temperature thereis a heating element 74 in an inlet 75 to the heat exchanger and atemperature sensor 76 in an outlet 77 from the heat exchanger.Additional temperature sensors can be placed in strategic positions inthe heat exchanger 70. The heat exchanger is insulated from thesurroundings to an extent which is suitable practically.

The conductivity cells 7a and 7b are connected to an electronic device78 which provides the required voltages and measuring devices in orderto carry out the conductivity measurement. The electronic device 78further comprises a subtraction circuit in order to arrive at adifference between the measured values from the cells 7a and 7b as wellas further possible arrangements for compensating the measured valuesfor temperature. These functions are preferably carried out with thehelp of a microcomputer. The microcomputer also converts the measuredvalues directly into the urea concentration in the sample solution.

The measured values may be corrected with respect to the prevailingtemperature according to known techniques. The temperature of thesolution in each sensor 7a and 7b is measured by temperature sensors 79and 80. Since however the flow at the measurement points is very low, itis difficult to obtain reliable temperature measurements.

In order to obtain a sufficient accuracy in the determination of theurea concentration, it is necessary that the temperature differencebetween the solutions in the cells 7a and 7b is kept very small, in theorder of magnitude of less than ±0,01° C. We have found that if thetubes 71 and 72 are sufficiently long and the temperature in the heatexchanger 70 is constant, sufficiently accurate measured values can beobtained.

In this case it is not necessary to temperature compensate eachconductivity value before subtracting them, but the temperaturecompensation can be made on the differential conductivity. Then, atemperature value which is the mean value of the two solutions can beused.

As the mean value can be used the temperature of the heat exchangingfluid in the heat exchanger but outside the coils 71, 72. Thistemperature can be obtained by a temperature sensor, such as temperaturesensor 110, shown in FIG. 9. The accuracy of the mean temperature neednot be as high as mentioned above, but can be ±0,4° C.

The fluid which is present within the heat exchanger 70 is preferablydialysis fluid from the conduit 1. In this way a relatively large flowcan pass through the heat exchanger 70, which ensures that thetemperature is substantially constant.

Alternatively a closed system can be used which is shown with dashedlines in FIG. 8, where a pump 81 circulates the fluid in an outer closedcircuit 82.

The preferred embodiment according to FIG. 8 can be additionallycomplemented by a drainage device 25, 27 according to FIG. 1 in the leftbranch, which thereby contains a separate valve arrangement or pump.

In another alternative the arrangement according to FIG. 8 is providedwith a cross-coupling which connects the urease column 10 with the tube72 and the delay conduit 22 with the tube 71. In this way themeasurement cells 7a and 7b respectively can be used alternately for theurease-column 10 and the delay conduit 22, whereby a reliablecalibration can be obtained.

By using the throttle device 53 and the pump 6, an overpressure can beachieved in the urea-sensor in the order of magnitude of 0,1 MPa,whereby a sufficiently high solubility for the carbon dioxide isobtained before the urease column 10 for the intended linear measurementrange up to 35 mM urea concentration. The reason for this linearity isabove all that the conversion is based on reaction (5), which is heavilydisplaced to the right due to the catalysis by urease and the surplus ofcarbon dioxide as well as a low pH-value.

FIG. 9 shows a more detailed flow scheme of the preferred embodiment ofthe urea sensor. The solution, enters from a dialysis machine via aninlet line 111 and enters a heater 112 and flows therefrom to a bubbleseparator 113. From there, the dialysate is conducted further on to aheat exchanger 114 and via an outlet line 115 to an outlet as shown byarrow 116.

The heater 112 comprises a heater rod 117 and a temperature sensor 118.Another temperature sensor 119 is positioned in the inlet line 111immediately before the heater 112 and measures the temperature of theinlet dialysate. Moreover, there is a power measuring device 120 formeasuring supplied power to the heater rod 117 via a suitable currentsource 121.

The inlet temperature of the dialysate in the line 111 is usuallybetween about 20-37° C., for example about 34° C. The heater rod 117 isadapted to rise the temperature to for example about 42° C., which issensed by a temperature sensor 110 positioned at heat exchanger 114.

By measuring the temperature difference between temperature sensor 110and temperature sensor 119 and supplied power from the current source121 by the power meter 120, the mass flow of dialysate through line 111can be determined (a so called thermal flow meter). Since the dialysatehas an essentially constant composition, its specific heat capacitivityand density is approximately constant and the amount of dialysate, whichis heated by the heater rod 117, can be determined as to its mass and/orvolume.

Since the rise in temperature by means of the heater rod 117 isrelatively slight, the accuracy of such a thermal flow meter will below. The flow meter can be used for sensing that the inlet fluid flowvia the inlet line 111 are within predetermined limits. Furtherpossibilities of use appears from the description below.

The bubble separator 113 comprises a chamber having relatively largecross section so that the flow velocity is low through the chamber froman inlet 122 at the lower end of the chamber to an outlet 123 at theupper end of the chamber. The bubble separator 113 is provided with apartition wall 124 and a second outlet 125 is adapted behind thepartition wall 124 in relation to the inlet 122 in order to protect theoutlet from turbulence. From outlet 125 a small partial quantity of thedialysate is taken out, a so-called sample solution of for example about1% or less, which partial amount constitutes the fluid which is analyzedas to the contents of urea (or any other substance of interest). Thus,the sample solution taken out via outlet 25 will be comparatively freefrom gas bubbles.

The sample solution taken out passes from outlet 125 via a line 126 to abranch point 127. Line 126 comprises an inlet coupling 128 for a bag 129enclosing a test solution, and a ground connection 130 for grounding thefluid in line 126. Moreover, there is an airation valve 131.Furthermore, line 126 may be winded a few revolutions around thedialysate line 115 as shown at 132 for equalizing possible temperaturedifferences between the fluids in the lines.

Coupling 128 is intended for cooperation with a connector 133. Whenconnector 133 is inserted in the coupling 128, a valve 134 in line 126before coupling 128 is closed so that the contents of the bag 129 passesthrough line 126 to the branch point 127 instead of the sample solution.

The bag 129 including test solution may comprise about 50-100 ml testsolution, which comprises urea in a known concentration. The testsolution can be colder than the temperature of the dialysate of thesample solution, and that is why the heat exchanger 132 heats the samplesolution to a certain degree. A volume of about 55 ml is sufficient forabout 20 minutes use of the test solution.

At the branch point 127, the sample solution from line 126 is dividedinto three branches. The first branch goes via a line 135 to a pump 136of peristaltic type. The second branch goes via a dummy volume 137 to asecond pump 138 of peristaltic type. The third branch goes via a line139 to a third pump 140 of peristaltic type.

From pump 136, the sample solution passes further on via a line 142 to aurease column 141 in the nature of a disposable article. The columncomprises a sufficient amount of urease and further material requiredfor transforming the urea contents of the sample solution to ammoniumions and bicarbonate ions as described above. The column 141 is shapedas a cartridge having a specific shape and with connections so that iteasily be connected to the urea sensor.

Before the urease column 141 there is supplied carbon dioxide gas to theline 142, for example in a T-coupling 143. The pressure of the suppliedgas is measured by a pressure meter 144. The amount of gas supplied iscontrolled by a valve arrangement as closer described below.

In the urease column 141, the urea contents of the solution isdecomposed into ammonium ions and bicarbonate ions. The transformedsolution is passed via a line 145 to a bubble separator 146. From theoutlet connection of the bubble separator, the solution passes furtheron via a valve 147 (closer described below) via a line 148 to a fourthpump 149.

All pumps 136, 138, 140, 149 are driven by a common motor 150 via acommon shaft or via any suitable transmission. The pumps are preferablyso-called peristaltic pumps. The pump 136 has preferably a slightlylarger capacity than the other pumps, which mutually have the samecapacity, which can be about 0,6 ml/min for the pumps 138, 140, 149 andabout 20% more for the pump 136. The pump 140 can have a differentand/or separate capacity.

During normal operation, a closed system prevails between the first pump136, line 142, urease column 141, line 145, bubble separator 146, viavalve 147, line 148 and pump 139. Thus, the difference in capacity(about 20%) between pumps 136 and 149 must be given off via the secondline 151 of the bubble separator for separated gases. Thus, the inletsolution via line 145 to the bubble separator 146 is divided in twoflows, 20% via line 151 and 80% via line 148. The major portion of thegas contents of the inlet line 145 passes out via the upper outlet toline 151.

The solution which passes out through line 148 to pump 149 is thus moreor less completely free from gases. This solution is conducted furtheron via a line 152 to a conductivity measurement cell 153 (below namedcondcell).

From the branch point 127 a partial amount of the sample solution ispassed via the volume 137 and the pump 138 and via a line 154 to asecond condcell 155. In this way there is obtained a time delay of saidpartial amount passing the volume 137, which is approximately equallylarge as the time delay for the partial amount which passes the ureasecolumn.

Before the solutions in the lines 152 and 154 reaches each respectivecondcell 153, 155, the solution passes through tubes, for example in theshape of heat exchanger coils 156, 157 positioned in the heat exchanger114 and thus surrounded by dialysis solution of relatively constanttemperature. The conditions are such that the solutions in the two coils156, 157 have passed through approximately the same volumes and lengthesof lines at their way to the coils 156, 157. Moreover, the coils arepositioned very tight adjacent to each other in the heat exchanger,which means that the temperature of the two solutions are equal or verysimilar, typically less than a difference of 0,01° C.

The difference in conductivity between the two solutions depends on thecontents of ammonium ions and bicarbonate ions in the catalyticallydecomposed solution, which thus can be measured and correlated to theurea contents.

Due to the supply of carbon dioxide, the relationship between the amountof urea and ammonium ions is linear over a large area sufficient for themeasurement of the present invention. Thus, no calibrations are requiredfor corrections for possible deviations from a linear relationship.

From the two condcells 153, 155, the solution passes further on via acommon line 158 to a pressure equalizing device 159. In line 158 thereis further placed a ground connection 160 in order to ensure that noelectric disturbances enter via the solution from the surroundings.

The pressure equalization device 159 comprises a container 161 having aninlet connection from line 158 and an outlet connection to an outletline 162 both preferably in the bottom 161. In the upper end of thecontainer there is a connection to a line 163, which terminates with apressure meter 165. The signal from the pressure meter 165 controls theopening degree of a valve 164 positioned in outlet line 162.

When solution enters the container 161 via line 158, the container isfilled and air passes out via line 163 and influences upon the pressuremeter 165. When a certain predetermined pressure prevails in thecontainer 161, valve 164 is opened and allows the solution to pass outvia line 162 and valve 164. Thus, a certain predetermined pressure ismaintained in container 161 and controlled via the remaining amount ofair in container 161 and the valve 164.

The remaining amount of air in container 164 operates as an air cushionwhich damps possible pressure oscillations which may have a tendence ofoccuring during for example a pump stroke. By means of device 159, anoverpressure is obtained in the lines between the pumps and up to device159. This overpressure means that possible remaining carbon dioxide inthe solution which passes via line 152 and other possible remaining gasbubbles in the solutions reaching the condcells will dissolve in theliquid. In this way problems in connection with the measuring in thecondcells are avoided. It is known that gas bubbles considerably disturbthe measurement in the condcells.

It is also possible to change the pressure of device 159 by closingvalve 164 so that the pressure increases temporary or intermittent. Achange of the conductivity value (especially an increase) at increasedpressure indicates error in the condcells, as described above.

The present invention uses double gas separators, a first separator 122for taking out a sample from the dialysis solution which is essentiallyfree from bubbles, and a second separator 146, which separates apossible excess of supplied carbon dioxide gas. Moreover, the extrasafety measure of an increased pressure is used. The solubility of gasin a liquid decreases with increased temperature, but the temperature isrelatively constant in the present invention.

From valve 164, the solution passes out to the outlet line 115 fordialysate solution and is delivered to a waste etc.

The third pump 140 is connected to the branch point 127 via line 139 andtakes out a test sample with constant flow speed, for example about 0,6ml/min. From pump 140 this sample solution passes via a line 166 to anoutlet 167 and via a valve 171 to the outlet line 115. In the outlet 167can be inserted a connector 170 which via a line 169 is connected to acollection bag 168. When the connector 170 is inserted in the outlet167, valve 171 is closed and all the solution in line 166 must pass tothe collection vessel 168.

Preferably pump 150 is driven with constant speed in order to take out aconstant partial amount of the dialysate solution passing through theentire urea sensor. Alternatively, pump 150 can be driven with a speedwhich is proportional to the amount of dialysate solution passing inoutlet line 115, for example with a proportion constant of 1:500 and isused for the purpose stated in European patent application 94.102383.0.In this case, motor 150 which drives pumps 136, 138, 140, 149 iscontrolled by a signal obtained from the dialysis machine so that saidproportionality constant is obtained. Alternatively, it is possible touse the measurement of the dialysis flow achieved via the heater rod 117and corresponding temperature sensors 110, 119.

Carbon dioxide gas is supplied via the T-connection 143 to the samplesolution as described above. The carbon dioxide gas is obtained from asuitable source 132 for carbon dioxide gas, which can be a carbondioxide cartridge or a device for generating carbon dioxide gas in situ.

The pressure from the source of carbon dioxide gas is controlled with aregulator 173 of known construction and the outlet pressure is monitoredby a pressure sensor.

Two valves 175, 176 are adapted in a line 177 leading from the pressureregulator 173 to said T-coupling 143. Valves 175, 176 are controlled sothat when one is open the other is closed. By controlling the frequencyfor opening and closing of said valves, the amount of carbon dioxide gaspassing valves 175, 176 is controlled if the pressure difference acrossthe valves is known. The pressure difference is measured by the pressuresensors 174 and 144.

The amount of carbon dioxide gas introduced in the branch point 143 iscontrolled so that a sufficient amount of carbon dioxide gas isintroduced while a too large excess is avoided.

Line 177 comprises a valve 178 connecting line 177 with container 159under certain operation conditions such as disinfection of the system.

There is a bypass-line 179 bypassing the urease column 141 when it istaken out from its holder.

Heat exchanger 114 comprises a cylindrical or rectangular container 180having a relatively large volume, for example 2 dl. The dialysatesurrounds the heat coils 156 and 157 so that they obtain the sametemperature as the dialysate and mutually the same temperature to a verylarge accuracy.

The condcells 153, 155 are positioned in the bottom of the container atthe outside of the container 180 as more closely appears from FIG. 10.The heat coils 156, 157 terminate in two openings 181 and 182 throughthe relatively thick bottom 183 of the container. At the outside of thebottom 183 (or in a recess in the bottom plate), the two condcells 153,155 are integrated to a single unit on the same substrate. Thus, it isassured that they adopt the same temperature. By having both cells inclose thermal prixmity, the accuracy required for the temperature sensoris reduced.

As shown in FIG. 11, the condcells comprise two plates 184, 185, whichare made for example of sapphir and comprise each a longitudinal concaverecess 187, 188, respectively. The plates are entirely symmetrical andare faced against each other and connected against each other withspring loaded attachment means (not shown in FIG. 11) so that a sealing186 between the sapphir plates is always loaded. The sealing 186 can bemade very thin and is made of a plastic material, which is inert inrelation to the dialysate, such as polypropylene.

Opposite the corresponding recess 187, 188 there are several goldelectrodes 189, 190, which extend out to the sides of the plate. Eachelectrode cooperates with a corresponding contact 191, only one of whichis shown in FIG. 11. There are in principal six equal electrodes 189,positioned symmetrically around a common electrode 190.

Between the two outer electrodes 189 and the electrode 190 there isapplied an electric current via a current generator. The voltage overthe two next following electrodes on each side is measured and gives twomeasurement signals which in principal should be equally large (possiblyafter correction due to different distances).

The gold electrodes 189, 190 can be applied by means of knowntechniques, such as plating or by means of ion assists or ion implant.Other known methods can also be used.

Since the plates are made of sapphir having a good thermal conductivity,only small temperature differences occur internally in the condcell. Ofcourse, also other conventional materials can be used.

The sample solution enters through holes 181, 182, respectively, to eachrecess 187, 188 and passes in the same direction to outlet holes 192,193 in order to obtain similar temperature conditions. It isalternatively possible that the flow through the recesses takes place inopposite directions.

In an alternative embodiment, each plate has two recesses and the platesare so positioned that each recess faces a corresponding recess in theother plate. A further alternative is to provide one plate with tworecesses while the other plate is plain and provided with theelectrodes. In all other respects, the operation is identical to theembodiment described above.

The plates 184, 185 are maintained against each other by means of springloaded attachment means 194 as shown in FIG. 10. There can be many suchattachment means distributed over the periphery of the plates in orderto give a uniform pressure distribution on the plates 184, 185 and thesealing 86 interposed therebetween. Also the contacts 191 contribute tokeeping the plates together.

The urea sensor described above requires cleaning and disinfection withregular intervalls in order to maintain a high accuracy. Disinfectionwith heat can be performed at the same time as the dialysis machinewhich the urea sensor is connected to. Disinfection can take place aftereach treatment or each day.

When the dialysis machine is disinfected, dialysate having increasedtemperature passes through the urea sensor, which is sensed by thetemperature sensors 119 and 110. Then, the operation of the urea sensoris also switched over to heat disinfection and the heater rod 117 heatsthe inlet dialysate further to a temperature of about 95° C. The ureasecolumn 141 has to be disconnected and the bypass line 179 is activatedsince the urease material does not withstand high temperatures. In thisway an effective heat disinfection of the urea sensor is obtained. Theurea sensor can also be cleaned by chemical means.

At disinfection (chemical and/or with heat) the valves are placed incertain positions as shown in FIG. 12 and FIG. 13. FIG. 12 showsdisinfection, phase one. Then, fluid passes from inlet 125 via line 126,valve 131, coil 132 to point 127. From point 127, the fluid passesfurther on via pump 138, line 154, coil 157, cell 155, line 158, vessel161 and valve 164 to the outlet 116. Moreover, the solution passes frompoint 127 via line 139, pump 140, line 166, connector 167 to valve 171.

Valve 180, which connects valve 171 to the outlet is, however, closed asis shown by the cross 181 in FIG. 12. Thus, the solution in line 166must pass via valve 171 and backwards in line 151 to the bubbleseparator 146 and then via valve 147, line 148, pump 149, line 152, coil156, cell 153 to the line 158 and further on to the outlet.

At the same time the solution passes from point 127 via line 135 andpump 136 to the line 142. Due to the flow conditions, the pressure atpoint 143 and the pressure at the inlet to the bubble chamber 146 areapproximately equal and that is why the solution does not pass throughthe bypass line 179 via line 145 to the bubble separator 146 but insteadpasses from the branch point 143 and backwards in line 177 (where it isnormally only carbon dioxide) and then via valve 178 and line 182 to theupper end of the chamber 159. In this first phase the main part of thesystem is disinfected except for valve 180 and line 145 and bypass line179.

In the second phase of the cleaning as shown in FIG. 13, valve 180 isswitched over so that it is open and valve 147 is switched forconnecting line 148 with a line 183 and to block the connection betweenthe bubble separator 146 and the line 148. By this measure, the line 145and the bypass line 179 are disinfected with a large flow, which gives ahigh heating (at heat disinfection). Moreover, valve 164 is closed whichmeans that the flow in line 182 changes direction and goes from chamber159 to valve 178 and the point 143.

The present invention has above been described with reference to severalembodiments shown on the drawings. The invention can however be variedand modified in many manners within the scope of the invention. Thedifferent single features shown in one or the other drawings can becombined in manners different from those shown on the drawings. Suchmodifications, which are obvious to a skilled person reading thisspecification, is intended to be within the scope of the invention. Theinvention is only limited by the appended patent claims.

We claim:
 1. A method for measuring the concentration of a decomposablecompound in a solution comprising adding a buffer in gaseous form tosaid solution, measuring the conductivity of said solution, decomposingsaid decomposable compound so as to produce a reacted solution,measuring the conductivity of said reacted solution, and calculating thedifferential conductivities between said solution and said reactedsolution so as to provide a measure of the concentration of saiddecomposable compound in said solution, wherein said buffer in gaseousform comprises carbon dioxide.
 2. The method of claim 1 wherein saidadding of said carbon dioxide to said solution comprises diffusing saidcarbon dioxide through a silicon tube into said solution.
 3. The methodof claim 1 wherein said decomposing of said decomposable compoundcomprises catalytically reacting said solution.
 4. The method of claim 3comprising catalytically reacting said solution with urease.
 5. Themethod of claim 1 wherein said calculating of said differentialconductivities comprises increasing the pressure of said solution inorder to increase the solubility of said carbon dioxide in saidsolution.
 6. The method of claim 5 including increasing said pressure ofsaid solution to about 0.1 MPa.
 7. The method of claim 1 wherein saidcalculating of said differential conductivities comprises decreasing thetemperature of said solution in order to increase the solubility of saidcarbon dioxide in said solution.
 8. The method of claim 7 includingdecreasing the temperature of said solution to about 25° C.
 9. Themethod of claim 1 wherein said measuring of said conductivity of saidsolution and said measuring of said conductivity of said reactedsolution are carried out by means of a single measurement cell.
 10. Themethod of claim 9 including measuring said conductivity of said solutionby connecting said single measurement cell at a location upstream ofsaid decomposing step and measuring said conductivity of said reactedsolution by connecting said single measurement cell at a locationdownstream of said decomposing step.
 11. The method of claim 9 includingmeasuring said conductivity of said solution by diverting a portion ofsaid solution from said decomposing step.
 12. The method of claim 1including diverting a portion of said solution into a flow path separatefrom said decomposing step, measuring said conductivity of said solutionby means of a first measurement cell in said separate flow path, andmeasuring said conductivity of said reacted solution by means of asecond measurement cell downstream of said decomposing step.
 13. Themethod of claim 12 including delaying the flow of said solution throughsaid separate flow path whereby the flow of said solution through saidseparate flow path to said first measurement cell and said flow of saidreacted solution to said second measurement cell take approximately thesame amount of time.
 14. The method of claim 1 wherein said decomposablecompound comprises urea.
 15. The method of claim 14 wherein saidsolution comprises bicarbonate ions.
 16. The method of claim 15 whereinsaid decomposing step comprises the following reaction:

    (NH.sub.2).sub.2 CO+CO.sub.2 +3H.sub.2 O→2NH.sub.4.sup.+ +2HCO.sub.3.sup.-.


17. The method of claim 15 wherein said reacted solution has a pH ofbetween about 6 and
 8. 18. The method of claim 17 wherein said reactedsolution has a pH of between about 6 and 7.4.
 19. A method for measuringthe concentration of a decomposable compound in a solution comprisingadding a buffer in gaseous form to said solution, dividing said solutioninto a first portion and a second portion, decomposing said decomposablecompound in said first portion of said solution so as to provide a firstreacted solution, equalizing the temperatures of said first reactedsolution and said second portion of said solution in a heat exchangerhaving a mean temperature, measuring the conductivities of saidtemperature equalized first reacted solution and second portion of saidsolution, and calculating the differential conductivity between saidfirst reacted solution and said second portion of said solution,temperature compensating said differential conductivity by means of themean temperature of said heat exchanger, and calculating saidconcentration of said decoposable compound from said compensateddifferential conductivity, wherein said buffer in gaseous form comprisescarbon dioxide.
 20. The method of claim 19 wherein said heat exchangercomprises a heat exchange fluid in heat exchange communication with saidfirst reacted solution and said second portion of said solution, andincluding providing said mean temperature of said heat exchanger bymeasuring the temperature of said heat exchange fluid.
 21. The method ofclaim 19 wherein said measuring of said conductivities comprisesmeasuring the conductivity of said temperature equalized first reactedsolution and said second portion of said solution with separateconductivity measurement cells.
 22. The method of claim 21 includingmeasuring said conductivity of said solution by diverting a portion ofsaid solution from said decomposing step.
 23. The method of claim 19wherein said decomposable compound comprises urea.
 24. The method ofclaim 23 wherein said decomposing of said decomposable compoundcomprises catalytically reacting said decomposable compound in saidsolution.
 25. The method of claim 24 wherein said catalytically reactingof said solution comprises reaction with urease.